With focusing devices, which include binoculars, monoculars, field glasses, and telescopes, there are different distances between the eyepiece lenses and eyeballs that are comfortable to the users due to the differences in their vision. Therefore, the eyepieces (the part that is in contact with eyes) of binoculars are often equipped with variable focusing devices, or “Twist Up devices”, that can adjust the distance between the eyepiece lenses and eyeballs.
A commonly used Twist Up device is basically composed of a Twist Up ring and a support ring that is often called a split-image suppressor ring. The Twist Up ring has a longer Twist Up adjustment groove, and the support ring is contained inside the Twist Up ring, and it is equipped with the Twist Up adjustment pins that are inserted into the above Twist Up adjustment groove. The Twist Up device moves the Twist Up adjustment pins inside the Twist Up adjustment groove along its circumference, and it twists up by moving the Twist Up ring against the support ring in the adjustment groove. In so doing, the distance between the eyepiece lenses and eyeballs is made larger or smaller by the distance that the Twist Up ring has moved.
The Twist Up adjustment groove in the conventional Twist Up ring is set in a straight line, diagonally from the opening at one end to the opening at the other end of the Twist Up ring. For this reason, when the conventional Twist Up device was operated by moving the Twist Up ring against the support ring so that the pin slides along the groove to a desired position. However, there is no means known in the art to fix the Twist Up ring at a fixed position. Consequently, when the Twist Up ring is pressed against the eyeballs to look at something, the pin tends to slide in the groove and the Twist Up ring consequently rotates, and it was not possible to maintain the desired position originally obtained by manipulating the Twist Up device. As a result, a focusing device with the conventional Twist Up devices known in the art fail to secure a proper distance between the eyepiece lenses and eyeballs which causes images to lose focus, difficulty in seeing desired objects and other inconveniences to the user.
In an attempt to resolve this limitation in the art, a Twist Up device has been developed with a retaining section using a spring-operated retaining ball in the support ring, which can be pushed against the Twist Up ring and held steady in the Twist Up adjustment groove of the Twist Up ring. With this Twist Up device, when the Twist Up ring is moved along the support ring in a set groove and the retaining ball comes to the set position in the retaining section and remains in the retaining section by the spring action. As a result, the retaining ball restricts the Twist Up ring from moving to the direction of the groove. Thus, this Twist Up device is able to prevent the movement of the Twist Up ring when the user was looking at something through the focusing device, such as a binocular or other device as mentioned above.
However, there are limitations to the art of Twist Up devices having a restraining ball and spring. The above retaining ball is held in place only the spring force. So, when a force applied to the focusing device is stronger than the spring force applied to the Twist Up device, the retaining ball comes off of the retaining section, and the Twist Up ring moves thereby failing to maintain the desired position obtained by manipulating the Twist Up device, which is a significant shortcoming.
Another limitation to Twist Up devices having a retaining ball and spring is the increased number of parts required for manufacturing the devices in addition to using the device. Additionally, a special manufacturing process is necessary to secure the above-mentioned retaining function. The increased number of parts complicates the manufacturing process, increases the manufacturing costs, and increase the number of components which may potentially fail. These are further shortcomings in the art.